Acifluorfen is a well known herbicide used widely in a variety of agricultural applications. Its chemical name is 5-(2-chloro-4-(trifluoromethyl)phenoxy)-2-nitrobenzoic acid and it is currently available from BASF Corporation under the trademark BLAZER.RTM.. U.S. Pat. Nos. 4,350,522, Re. No. 31,455 and Re. No. 31,731 all relate to the production of acifluorfen and its related salts. The synthetic route used to produce acifluorfen is illustrated below. The initial step in the synthesis is the reaction between 3-hydroxybenzoic acid (mHBA) and 3,4-dichlorobenzyltrifluoride (3,4-DCBTF) in the presence of potassium hydroxide (KOH) and dimethyl sulfoxide (DMSO) to form 3-(2-chloro-4-(trifluoromethyl)phenoxy)benzoic acid with a small amount of 3-(2-chloro-5-(trifluoromethyl)phenoxy)benzoic acid. The product is then nitrated in a mixture of HNO.sub.3 /H.sub.2 SO.sub.4 /acetic acid/acetic anhydride, precipitated by the addition of water, and filtered to provide crude acifluorfen: ##STR1##
Unfortunately, synthetically produced acifluorfen very often contains a fairly high percentage of isomeric impurities. It is conceivable that acifluorfen may be utilized as an intermediary in the synthesis of other compounds, and therefore it is chemically important to have a very pure acifluorfen starting product so as to produce high final yields of other chemical products.
Three compounds in particular should be removed from the manufactured acifluorfen product. The nitration of 3-(2-chloro-4-(trifluoromethyl)phenoxy)benzoic acid (unnitrated intermediate) produces, in addition to acifluorfen, 3-(2-chloro-4-(trifluoromethyl)phenoxy)-2-nitrobenzoic acid (hereinafter 2'-NO.sub.2 isomer). Also produced in this synthesis is 3-(2-chloro-4-(trifluoromethyl)phenoxy)-4-nitrobenzoic acid (hereinafter 6'-NO.sub.2 isomer). The nitration of the 3-(2-chloro-5-trifluoromethyl) phenoxy)benzoic acid, present as a byproduct in the synthesis route, produces 5-(2-chloro-5-(trifluoromethy1)phenoxy)-2-nitrobenzoic acid (hereinafter 5-CF.sub.3 isomer): ##STR2##
Typically, the crude acifluorfen contains about 80-85% of acifluorfen, 9-10% of the 2'-NO.sub.2 isomer, 3-4% of the 6'-NO.sub.2 isomer, and 4-5% of the 5-CF.sub.3 isomer and less than 1% unnitrated intermediate on a dry basis. These percentages may of course vary depending upon the particular reaction components and parameters utilized by those skilled in the art. In any event, there remains the problem of a substantial number of the aforementioned impurities present in the manufactured acifluorfen.
Many chemical purification processes involve dissolution of the preferred compound together with its impurities in a solvent, and then recrystallization of the desired compound. Typically, the final yield of the desired compound is smaller via the purification route. For this reason, care must be taken so as not to sacrifice yield for purity.
There presently exists a need in the art to purify acifluorfen so as to remove undesirable isomers. At the same time, there is also a need to maintain substantially high final yields of the purified acifluorfen product.